<!-- Author: Phrogz -->
<!-- Origin: http://phrogz.net/tmp/canvas_zoom_to_cursor.html -->

<!DOCTYPE HTML>
<html lang="en"><head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
	<title>Zooming via HTML5 Canvas Context</title>
	<style type="text/css" media="screen">
		body { background:#eee; margin:1em; text-align:center; }
		canvas { display:block; margin:1em auto; background:#fff; border:1px solid #ccc }
	</style>
</head><body>
<p>Showing how to use transform methods on the HTML5 Canvas Context to selectively zoom in and out. Drag to pan. Click to zoom at that location. Shift-click to zoom out. Mousewheel up/down over the canvas to zoom in to/out from that location.</p>
<p>By redrawing the canvas at different scales the strokes remain smooth during zooming. For example, zoom in on the earring or circle on the forehead to see fine detail.</p>
<canvas></canvas>
<p id="footer">Copyright © 2011 <a href="mailto:!@phrogz.net">Gavin Kistner</a>. Written to support <a href="http://stackoverflow.com/questions/5189968/zoom-to-cursor-calculations/5526721#5526721">this Stack Overflow answer</a>.</p>
<script type="text/javascript" charset="utf-8">
	var canvas = document.getElementsByTagName('canvas')[0];
	canvas.width = 800; canvas.height = 600;
	var gkhead = new Image;
	var ball   = new Image;
	window.onload = function(){		
		var ctx = canvas.getContext('2d');
		trackTransforms(ctx);
		function redraw(){
			// Clear the entire canvas
			var p1 = ctx.transformedPoint(0,0);
			var p2 = ctx.transformedPoint(canvas.width,canvas.height);
			ctx.clearRect(p1.x,p1.y,p2.x-p1.x,p2.y-p1.y);

			// Alternatively:
			// ctx.save();
			// ctx.setTransform(1,0,0,1,0,0);
			// ctx.clearRect(0,0,canvas.width,canvas.height);
			// ctx.restore();

			ctx.drawImage(gkhead,200,50);

			ctx.beginPath();
			ctx.lineWidth = 6;
			ctx.moveTo(399,250);
			ctx.lineTo(474,256);
			ctx.stroke();

			ctx.save();
			ctx.translate(4,2);
			ctx.beginPath();
			ctx.lineWidth = 1;
			ctx.moveTo(436,253);
			ctx.lineTo(437.5,233);
			ctx.stroke();

			ctx.save();
			ctx.translate(438.5,223);
			ctx.strokeStyle = '#06c';
			ctx.beginPath();
			ctx.lineWidth = 0.05;
			for (var i=0;i<60;++i){
				ctx.rotate(6*i*Math.PI/180);
				ctx.moveTo(9,0);
				ctx.lineTo(10,0);
				ctx.rotate(-6*i*Math.PI/180);
			}
			ctx.stroke();
			ctx.restore();

			ctx.beginPath();
			ctx.lineWidth = 0.2;
			ctx.arc(438.5,223,10,0,Math.PI*2);
			ctx.stroke();
			ctx.restore();
			
			ctx.drawImage(ball,379,233,40,40);
			ctx.drawImage(ball,454,239,40,40);
			ctx.drawImage(ball,310,295,20,20);
			ctx.drawImage(ball,314.5,296.5,5,5);
			ctx.drawImage(ball,319,297.2,5,5);
		}
		redraw();
		
		var lastX=canvas.width/2, lastY=canvas.height/2;
		var dragStart,dragged;
		canvas.addEventListener('mousedown',function(evt){
			document.body.style.mozUserSelect = document.body.style.webkitUserSelect = document.body.style.userSelect = 'none';
			lastX = evt.offsetX || (evt.pageX - canvas.offsetLeft);
			lastY = evt.offsetY || (evt.pageY - canvas.offsetTop);
			dragStart = ctx.transformedPoint(lastX,lastY);
			dragged = false;
		},false);
		canvas.addEventListener('mousemove',function(evt){
			lastX = evt.offsetX || (evt.pageX - canvas.offsetLeft);
			lastY = evt.offsetY || (evt.pageY - canvas.offsetTop);
			dragged = true;
			if (dragStart){
				var pt = ctx.transformedPoint(lastX,lastY);
				ctx.translate(pt.x-dragStart.x,pt.y-dragStart.y);
				redraw();
			}
		},false);
		canvas.addEventListener('mouseup',function(evt){
			dragStart = null;
			if (!dragged) zoom(evt.shiftKey ? -1 : 1 );
		},false);

		var scaleFactor = 1.1;
		var zoom = function(clicks){
			var pt = ctx.transformedPoint(lastX,lastY);
			ctx.translate(pt.x,pt.y);
			var factor = Math.pow(scaleFactor,clicks);
			ctx.scale(factor,factor);
			ctx.translate(-pt.x,-pt.y);
			redraw();
		}

		var handleScroll = function(evt){
			var delta = evt.wheelDelta ? evt.wheelDelta/40 : evt.detail ? -evt.detail : 0;
			if (delta) zoom(delta);
			return evt.preventDefault() && false;
		};
		canvas.addEventListener('DOMMouseScroll',handleScroll,false);
		canvas.addEventListener('mousewheel',handleScroll,false);
	};
	gkhead.src = 'http://phrogz.net/tmp/gkhead.jpg';
	ball.src   = 'http://phrogz.net/tmp/alphaball.png';
	
	// Adds ctx.getTransform() - returns an SVGMatrix
	// Adds ctx.transformedPoint(x,y) - returns an SVGPoint
	function trackTransforms(ctx){
		var svg = document.createElementNS("http://www.w3.org/2000/svg",'svg');
		var xform = svg.createSVGMatrix();
		ctx.getTransform = function(){ return xform; };
		
		var savedTransforms = [];
		var save = ctx.save;
		ctx.save = function(){
			savedTransforms.push(xform.translate(0,0));
			return save.call(ctx);
		};
		var restore = ctx.restore;
		ctx.restore = function(){
			xform = savedTransforms.pop();
			return restore.call(ctx);
		};

		var scale = ctx.scale;
		ctx.scale = function(sx,sy){
			xform = xform.scaleNonUniform(sx,sy);
			return scale.call(ctx,sx,sy);
		};
		var rotate = ctx.rotate;
		ctx.rotate = function(radians){
			xform = xform.rotate(radians*180/Math.PI);
			return rotate.call(ctx,radians);
		};
		var translate = ctx.translate;
		ctx.translate = function(dx,dy){
			xform = xform.translate(dx,dy);
			return translate.call(ctx,dx,dy);
		};
		var transform = ctx.transform;
		ctx.transform = function(a,b,c,d,e,f){
			var m2 = svg.createSVGMatrix();
			m2.a=a; m2.b=b; m2.c=c; m2.d=d; m2.e=e; m2.f=f;
			xform = xform.multiply(m2);
			return transform.call(ctx,a,b,c,d,e,f);
		};
		var setTransform = ctx.setTransform;
		ctx.setTransform = function(a,b,c,d,e,f){
			xform.a = a;
			xform.b = b;
			xform.c = c;
			xform.d = d;
			xform.e = e;
			xform.f = f;
			return setTransform.call(ctx,a,b,c,d,e,f);
		};
		var pt  = svg.createSVGPoint();
		ctx.transformedPoint = function(x,y){
			pt.x=x; pt.y=y;
			return pt.matrixTransform(xform.inverse());
		}
	}
</script>
</body></html>
